1. Field of the Invention
The present invention relates to a toroidal type continuous variable transmission system used as, e.g., a transmission of an automobile.
2. Related Background Art
A toroidal type continuous variable transmission system used as, for example, a transmission of an automobile has been known as described in, e.g., Japanese Patent Unexamined Publication Nos. Hei. 1-169169 and Hei. 1-312266. The toroidal type continuous variable transmission system comprises a toroidal type continuous variable transmission including a power roller which is rotatably interposed at an angle between input and output disks while remaining in contact with them; and a planetary gear mechanism connected to the output disk. The planetary gear mechanism comprises first and second planetary gear sets having sun gears connected to the output disk; a first power transmission mechanism which fixes a given element of the first planetary gear set to thereby selectively obtain a rotational force opposite to the direction of the output disk and transmits the thus-obtained rotational force to the second planetary gear set and an output shaft; and a second power transmission mechanism which connects a given element of the second planetary gear set to the input disk to thereby selectively obtain a rotational force opposite to the direction of the output disk and transmits the thus-obtained rotational force to the output shaft.
More specifically, the toroidal type continuous variable transmission system comprises a single cavity toroidal type continuous variable transmission and a set of two-staged planetary gears. A given element of the first planetary gear set is fixed by actuation of the first power transmission mechanism, thereby transmitting the torque of the output disk of the toroidal type continuous variable transmission to the output shaft by way of the first planetary gear set so as to cause a rotation in the direction opposite to the direction in which an input shaft rotates. As a result, there is obtained a first forward movement mode.
In the first mode, while the toroidal type continuous variable transmission is set in the maximum speeding-up position, the first power transmission mechanism is brought into an inactive state. A second power transmission mechanism is actuated in place of the first power transmission mechanism, thereby fixing a given element of the second planetary gear set. As a result, the torque of the input shaft is transmitted to the output shaft not by way of the toroidal type continuous variable transmission but directly by way of the second planetary gear set, thus realizing a second forward movement mode, or what is called a power circulation state in which part of the torque is returned to the input shaft by way of the second planetary gear set and the toroidal type continuous variable transmission.
In a continuously variable transmission described in Japanese Patent Examined Publication No. Hei 6-21625, the transmission comprises a double-cavity toroidal type continuous variable transmission and a two-staged planet gear mechanism. A planet carrier of a first planetary mechanism and a sun gear of a second planet gear mechanism are actuated by means of an output shaft of the continuously variable transmission. A sun gear of the first planetary gear mechanism and a ring gear of the second planetary gear mechanism are actuated by means of an engine. A ring gear of the first planetary gear mechanism acts as a low-speed output section, and the planetary carrier of the continuously variable transmission acts as a high-speed output section.
Therefore, to maintain an automobile in a standstill state without rotating a final drive shaft, there is prevented transmission of torque to the ring gear, which would otherwise be caused when the rotational speed of the planet carrier of the first planet gear mechanism cancels the rotation of the sun gear A transmission gear ratio of the transmission is thus set. Namely, by means of a mechanism for acquiring a differential component of each component of the planet gear mechanism, the automobile can be maintained in a standstill state without use of a start clutch, so long as the differential component is adjusted to zero. When the automobile starts, the transmission gear ratio of the transmission is gradually increased in such a manner that torque is gradually transmitted to the ring gear.
However, the conventional toroidal type continuous variable transmission systems described in Japanese Patent Unexamined Publication Nos. Hei. 1-169169 and Hei. 1-312266 require incorporation of two sets of planetary gear mechanisms, thus rendering the structure of the system complex and adding to cost. Further, the systems requires large installation space. Furthermore, a single cavity toroidal type continuous variable transmission suffers an inferior power transmission efficiency and cannot transmit great torque.
In a case where a mechanical loading cam is incorporated into a gear neutral system described in Japanese Patent Examined Publication No. Hei. 6-21625, an input disk is pressed against an output disk by way of a power roller. Depending on a transmission gear ratio, torque which is smaller than engine torque is input to a variator, whereas 100% of engine torque is input to the mechanical loading cam, thereby causing excessively great pressing force. In such a case, 100% of engine torque is input to the mechanical loading cam, whilst torque greater than the engine torque is input to the variator, thus producing excessively small pressing force. In the event of excessively great pressing force, the efficiency of power transmission is deteriorated, thus disabling transmission of great torque. In contrast, in the event of excessively small pressing force, the pressing force becomes insufficient, thus causing slippage. Since there is used a two-staged planetary gear set is used, as in the case of the previous example, the system becomes bulky.